This application claims the priority of German patent document 198 19 937.6, filed May 5, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to an electronic engine control system for an internal-combustion engine.
An engine control system of this type is used in modern internal-combustion engines for automatically controlling the operating performance of the internal-combustion engine. For example, such an engine control system interacts with a fuel injection system, a throttle valve in the air intake region of the internal-combustion engine, an exhaust gas recirculation system and an exhaust gas cleaning system. In addition, with a correspondingly constructed engine control system, the air/fuel mass ratio of the mixture fed to the combustion can be varied; in particular, a change-over can be permitted between rich and lean operations of the internal-combustion engine.
Modern internal-combustion engines (such as direct-injection gasoline engines, direct-injection diesel engines, common-rail injection diesel engines) may be equipped with an exhaust gas recirculation system in order to reduce the emission of pollutants and reduce the fuel consumption. In such systems,a portion of the engine exhaust gases is returned to the combustion mixture. Such an external exhaust gas recirculation changes the air-fuel mass ratio of the mixture charged into the combustion space and therefore has considerable influence on the combustion taking place there. As a result, the composition of the exhaust gases is changed, and the power and the smooth operation of the engine are influenced. In addition to being influenced by such an external exhaust gas recirculation, the operating performance of the engine can also be affected by internal exhaust recirculation. Such internal exhaust gas recirculation is performed, for example, by providing a dead volume which cannot be driven out of the combustion space by a piston of a cylinder. In addition, during charging of the mixture into the combustion space, an overlap between the closing movement of the exhaust valves and the opening movement of the intake valves leads to a return flow of exhaust gases already displaced from the combustion space into the exhaust gas system.
In order to obtain optimal values for engine performance, particularly for fuel consumption, pollutant emission and engine efficiency, the fraction of exhaust gases which exists in the combustion space before the next combustion operation must be known. only then, can the mixture composition in the combustion space be determined with sufficient precision and, as a result, by means of a suitable automatic control or control, the operating performance of the engine can be influenced.
The fraction of exhaust gas which remains in the combustion space before the next combustion (called a residual gas fraction) is taken into account with respect to characteristic diagrams in the case of conventional engine control systems. For this purpose, characteristic diagrams stored in a memory of the engine control system represent experimental values for adjustment of the exhaust gas recirculation system or of a mixing valve which influences the exhaust gas quantity externally fed to the combustion, as a function of different load condition of the internal-combustion engine. Because both internal and external exhaust gas recirculation can change during constant engine operation in order, for example, to be able to ensure a smooth engine running performance, considerable safety factors must be taken into account. As a result, the influence of the residual gas fraction by means of characteristic diagrams can be considered only in a relatively rough manner.
Other methods, which measure the residual gas fraction in the combustion space relatively precisely, have high equipment-related expenditures, and can therefore be used only in experiments within the scope of the engine development.
One object of the present invention, therefore, is to provide a system which can determine the residual gas fraction in the combustion space, at low expenditures.
These and other objects and advantages are achieved by the present invention, in which the residual gas fraction is determined exclusively from a conventionally measurable air/fuel mass ratio in the exhaust gases and from an air/fuel mass ratio adjusted by the engine control system in the carbureted fuel. Thus, for the system according to the invention, a normally existing engine control system can be used. By means of such an engine control system, an already existing signal value for the air/fuel mass ratio in the carbureted fuel is detected and is combined with a signal value for the air/fuel mass ratio in the exhaust gas sensed by a sensor system. Such a sensor system also already exists in internal-combustion engines with a modern exhaust gas cleaning system. The engine control system according to the invention can therefore easily be integrated into an existing engine control system.
In a preferred embodiment of the invention, the engine control system can influence the engine operation as a function of the signal value correlating with the residual gas fraction, when the engine control unit detects the existence of steady-state or quasi-steady-state engine operation and/or engine operation with an extrastoichiomeetric air/fuel mass ratio in the carbureted fuel. These measures are based in particular on the recognition that, in extrastoichiometric engine operation, either the fuel mass in the exhaust gas after combustion is essentially equal to zero (overstoichiometric air/fuel mass mixture in the carbureted fuel during lean operation) or the air or oxygen mass in the exhaust gas after the combustion is equal to zero (understoichiometric air/fuel mass ratio in the carbureted air in the rich operation). In addition, the invention takes into consideration that, in the case of an extrastoichiometric operation of the engine, because of the internal and external exhaust gas recirculation, HC (in the case of a rich understoichiometric operation) or O.sub.2 (in the case of a lean overstoichiometric operation) builds up in the combustion space. That is, the mixture available in the combustion space for the combustion on its own becomes increasingly richer or leaner within certain limits.
Under these conditions, the following therefore applies to the fuel: EQU m.sub.kz (k)=m.sub.kf (k)+r.sub.R m.sub.ka (k-1)
wherein: ##EQU1##
Analogously, the following applies to the air: EQU m.sub.lz (k)=m.sub.lf (k)+r.sub.R m.sub.la (k-1)
wherein: ##EQU2##
The following legend applies to all formulas and equations contained in the description:
m.sub.lz =air mass in the cylinder combustion space PA1 m.sub.lf =air mass in the carbureted fuel PA1 m.sub.la =air mass in the exhaust gas PA1 m.sub.kz =fuel mass in the cylinder combustion space PA1 m.sub.kf =fuel mass in the carbureted fuel PA1 m.sub.ka =fuel mass in the exhaust gas PA1 r.sub.R =residual gas fraction PA1 .DELTA.=stoichiometric air/fuel mass ratio PA1 .phi..sub.z =air/fuel mass ratio in the mixture contained in the cylinder combustion space or in the exhaust gas PA1 .phi..sub.f =air/fuel mass ratio in the carbureted fuel PA1 k=discrete point in time in the combustion space shortly before a combustion PA1 k-1=point in time k of the preceding combustion cycle
Since the indicated equation system is not linear in the range of stoichiometric engine operation, an analysis of the residual gas fraction is expedient only in the case of an extrastoichiometric air/fuel mass ratio in the carbureted fuel.
According to another feature of a preferred embodiment of the engine control system according to the invention, in the case of rich operation of the internal-combustion engine, the signal value correlating with the residual gas fraction may represent a value calculated analogously to the equation: ##EQU3##
This measure is based on the recognition that the following applies to a rich operation with constant operating conditions, particularly with a constant engine load: ##EQU4##
By dividing the above-mentioned equation by the air mass in the carbureted fuel (m.sub.lf), the following is obtained for the quasi-steady-state case: ##EQU5##
In this case, it was taken into account that, during rich operation (with an understoichiometric air/fuel mass ratio in the carbureted fuel after the combustion), no air or no oxygen is present in the exhaust gas, so that the following applies: EQU m.sub.la (k)=m.sub.la (k-1)=0
as well as: EQU m.sub.lf (k)=m.sub.lf (k-1)=m.sub.lz (k)=m.sub.lz (k-1)=const.
A Z-transformation results in the following: ##EQU6##
as well as: ##EQU7##
The limit value principle will then result in: ##EQU8##
Therefore, the following is obtained for the air/fuel mass ratio in the cylinder combustion space: ##EQU9##
In this regard, it should be noted that the air/fuel mass ratio in the cylinder combustion space (.phi..sub.z) does not change as a result of combustion. Thus, the air/fuel mass ratio existing in the exhaust gases is equal to the air/fuel mass ratio in the combustion space before the combustion; and in the equations, .phi..sub.z is the air/fuel mass ratio in both the combustion space and the exhaust gas.
The above-mentioned equation results in a conversion according to the residual gas fraction: ##EQU10##
Corresponding to another embodiment of the engine control system according to the invention, during lean operation of the internal-combustion engine, the signal value correlating with the residual gas fraction may represent a value calculated analogously to the equation: ##EQU11##
For lean operation (with an overstoichiometric air/fuel mass ratio in the carbureted fuel), considerations apply which are analogous to the approach shown for the rich operation: EQU m.sub.lz (k)-r.sub.R.multidot.m.sub.kz (k-1).multidot..DELTA.=r.sub.R.multidot.m.sub.kz (k-1)
Dividing this equation by the fuel mass in the carbureted fuel (m.sub.kf) results in the following: ##EQU12##
in which case the following applies for lean operation:
m.sub.ka (k)=m.sub.ka (k-1)=0
as well as: EQU m.sub.kf (k)=m.sub.kf (k-1)=m.sub.kz (k)=m.sub.kz (k-1)=const.
For this case, a Z-transformation results in: ##EQU13##
as well as: ##EQU14##
Then the limit value principle results in: ##EQU15##
The air/fuel mass ratio in the cylinder combustion space is independent of the combustion, and therefore equal to the air/fuel mass ratio in the exhaust gas. The following therefore applies: ##EQU16##
The above-mentioned equation will then result in a conversion according to the residual gas fraction: ##EQU17##
In a particularly advantageous embodiment of the engine control system according to the invention, the sensor system may have a .lambda.-probe arranged behind the combustion space or behind the engine in an exhaust gas system. By means of such a .lambda.-probe, the air/fuel mass ratio of the exhaust gases can be determined. Such a .lambda.-probe may already exist in the case of internal-combustion engines with a modern exhaust gas cleaning system.
In an advantageous further development of the engine control system according to the invention, the sensor system for each combustion space of the engine (that is, for each cylinder), may have a sensor, particularly a lambda probe, for the air/fuel mass ratio of the exhaust gases flowing out of the combustion space. In conventional engines, the exhaust gas recirculation pipe at a point leads into a collector feed for all cylinders of the engine. Thus, as a result of the existing flow conditions (for example, pulsations, flow deflections), the returned exhaust gases may not be uniformly mixed with the taken-in fresh air, so that a different air/fuel mass ratio exists in each combustion space, which can adversely affect the smooth running of the engine. By means of the invention, however, it is possible to determine the air/fuel mass ratio of the exhaust gases emerging individually from each combustion space for the current operating condition of the engine. Based on this knowledge of the air/fuel mass ratios in each of the combustion chambers, exhaust gas recirculation can then be controlled by the engine control system so that the same air/fuel mass ratio exists in every combustion space.
In an advantageous embodiment of this further development of the engine control system according to the invention, the quantity of recirculated exhaust gas can be varied by triggering an adjusting valve in an exhaust gas recirculation system as a function of the combustion space to be currently filled. For example, in the case of a 4-cylinder engine having four combustion chambers I, II, III and IV, the engine control system may determine that, in a certain operating condition of the engine, the current air/fuel mass ratio in combustion space I is lower than; in combustion spaces II and III is equal to; and in combustion space IV is higher than the air/fuel mass ratio provided by the engine control system for this operating condition of the engine. In this case, the adjusting valve of the exhaust gas recirculation system is controlled by the engine control system such that the fresh air--fuel--exhaust gas mixture fed to combustion spaces I to IV is varied as a function of the respective combustion space I to IV to be filled therewith. In this way, uniform distribution of the exhaust gas recirculation rate or of the air/fuel mass ratio can be achieved in all combustion spaces by means of a single adjusting valve.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.